Quantitative assessment of the sorption of cadmium and lead by bacteria, ferrihydrite, and bacteria-ferrihydrite composites (Bacillus subtilis, Escherichia coli)

Emerging as a central topic of environmental geochemistry is the geochemical characterization of reactions that occur at solid surfaces involving dissolved inorganic contaminants. The objectives of this work therefore were to quantify, in a comparative manner, the relative Cd2+ and Pb2+ sorptive properties of bacteria, ferrihydrite, and bacteria—ferrihydrite composite solid surfaces. This included a comparison of two bacteria (Bacillus subtilis and Escherichia coli) that differ in cell wall format (Gram-positive and Gram-negative, respectively). Regardless of the sorbent system, Pb2+ always demonstrated a higher affinity (KS_M) and sorption capacity (S_max) than Cd2+. For Cd2+ and Pb2+, the bacteria and bacteria—ferrihydrite composites demonstrated higher KS_M values than the bare ferrihydrite solids. Despite this phenomenon, ferrihydrite was capable of attaining higher S_max values compared to the bacteria—ferrihydrite composites or the lone bacteria. The results indicate that the sorptive properties of bacterial—ferrihydrite composite solids are non-additive in their sorption behavior, owing to the masking of surface functional groups on each neighboring sorbent that participates in the composite sorbent solid.